Switched hot swap controller

ABSTRACT

The invention describes an electric circuit ( 100 ), a method and a computer program for hot-swapping an electronic board in a telecommunication system, where the increase in current in the electric circuit is controlled by a microcontroller ( 130 ) switching a power transistor in a switching circuit ( 150 ) so as to gradually increase the capacitor voltage for the electronic board. The current level is measured either in the microcontroller ( 130 ) itself or in an external current sense circuit ( 140 ) and compared to a maximum current level.

TECHNICAL FIELD

The present invention relates to the field of hot swap controllercircuits using an external current limiting transistor.

BACKGROUND OF THE INVENTION

For quite a number of years the technology of hot swapping of electriccomponents has been known in the computer and the telecommunicationfields.

Hot swapping means adding or removing electric components into anexisting computer or telecommunication architecture without having toperform a power shut-down of the whole architecture first.

In the network technology fields, this technology provides thepossibility of adding or removing network adapters to or from abackplane, while the backplane is still delivering power to othercomponents in the network. Likewise, hard drive units in a redundantbackup system, such as a RAID (Redundant Array of Independent Disks),can be plugged in or taken out from the server machine, while the serveris still running.

The hot swap technology is being used in the computer field in order toinsert different kinds of adapters onto a motherboard on a computer,while it is still being provided with power or as external devicesplugged into USB (Universal Serial Bus)- or FireWire-ports belonging toa computer.

Hot-swapping in telecommunication applications allows for exampleinsertion or removal of line cards to or from a backplane in a basestation, while the backplane is still delivering power to other linecards connected to it. Thus system upgrades, maintenance and repair areperformed much faster with minimum disturbance to users of thetelecommunication network.

One common problem when hot-swapping units into an existing computer- ortelecommunications system is the occurrence of large inrush currents,which can exceed the operating current of the power supply and thusdamage either the computer- or telecommunication system or the componentitself, or both.

The reason for this put simply is that the component to be added to thesystem often presents a high capacitive load to the power source of thesystem and large capacitors need time to be loaded. However, during thesudden current surge caused by the switch-on of the power sourcecircuit, the large capacitor acts as a short circuit, thus leading to alarge inrush current going through the circuit of the component to beadded to the computer- or telecommunication system.

Similar phenomena are observed when such modules are removed from thecomputer- or telecommunication system.

The first and easiest way of dealing with inrush currents are the use ofdiscrete components in the form of thermistors whose resistance iscurrent dependent and increasing or decreasing with increasing current,so called PCTs and NTCs. Thus, during switch-on procedure of the powersupply, a thermistor is warmed up by the current flowing through it andslowly allows the current to rise, when a large capacitive load is addedto the power supply circuit. In this way, the initial rapid current risedue to the addition of the capacitive load is slowed down.

A disadvantage of the thermistor solution is its inherent slow responseto current transients and might not be desirable in an environment,where modules are frequently inserted or removed from for example atelecommunication system.

Thus, in addition to thermistors, rapid changes have to be responded toby fuses or fault protection devices, where the fuse add a voltage dropto the power path, which is generally not desired in these applications.Also, the thermistors themselves add a voltage drop to the power path.

A different solution still using discrete components is the use ofdiscrete MOSFETS, which provide low drain-to-source resistance Rds(on)and act almost as an ideal switch.

MOSFETs require low voltages to operate and can be switched on or offrapidly in order to respond rapidly to voltage changes.

The disadvantage of discrete MOSFETs for protection against inrushcurrents is the additional circuitry in the form of resistors andcapacitors necessary to control the current rise time and differentfault conditions, such as overcurrent.

Normally, such discrete MOSFETs are expensive and difficult to optimizeaccording to the respective applications.

Also, discrete MOSFETs have a parasitic diode connected from the drainto the source, which can lead to current backflow, when the outputvoltage on the device is higher than the input voltage.

The object of the present invention is to rectify some of thedisadvantages with known technology described above.

SUMMARY OF THE INVENTION

The object of the invention is achieved by an electric circuit forhot-swapping of an electronic arrangement, which includes circuitrywhich provides a current charging a capacitor on the electronicarrangement, a circuitry for controlling the increase in current whichcharges the capacitor on the electronic arrangement where said electriccircuit additionally includes circuitry which switches the currentcharging the capacitor and a processing unit which controls the timingof the switching of the current in the electric circuit.

In one embodiment of the invention, the circuitry for controlling theincrease in current which charges the capacitor on the electronicarrangement is a circuitry adapted for releasing the energy storedduring switching of the above current charging the capacitor in the formof another current additionally charging the capacitor. This circuitrymay include, for example, an inductor.

The inductor would in effect through the releasing of the currentcontrol the voltage charging the capacitor on the electronic circuitboard when the discrete transistor is switched from the “ON” to the“OFF” state.

In another embodiment of the invention, the circuitry for switching thecurrent charging the capacitor on the electronic board is a discretetransistor. However, other components for switching the current could beused, such as integrated circuits.

Also, the circuitry for switching the current may include a drivingcircuit for said discrete transistor in the form of a low-ohmic sourcefor the gate charge of the discrete transistor and a rectifier circuitin the form of a diode circuit. The function of the diode circuit is to“free wheel” or release the current from the energy stored in theinductor described below.

In yet another embodiment of the invention, the rectifier circuit mayinstead of a diode circuit include a transistor circuit or some othercircuitry performing an equivalent function.

In one other embodiment of the invention, the circuitry for controllingthe timing of the switching of the current charging the capacitor mayinclude a microprocessor, an ASIC, an FPGA or some other suitablecircuit which is either pre-programmed or programmable.

From the scalability and flexibility point of view, the use of aprogrammable microprocessor to control the timing of the switching ofthe current is preferable.

Depending on the source voltage a system is delivering to the electronicboard and the expected load capacitance of the electronic board, asimple adaptation of the program used in the microprocessor to thechanged desired signal levels and switching times is sufficient in orderto achieve the hot-swapping function. Thus, in comparison to existinghot-swapping systems, the electric circuit according to the presentinvention is easy to optimize and to configure. Also since the switchingof the discrete transistor is controlled by a microcontroller discretetransistors are not critical components in the system anymore.

The controlled switching of the discrete transistor also introduces theadvantage that less power is dissipated when the transistor is turned onand that the dissipation can be optimized by adequate programming of theswitching sequence in the microcontroller. As a consequence, transistorswith much lower SOA (Safe Operating Area) can be used than previously.This means implicitly that cheaper transistor components can be usingfor the switching transistor.

In yet another embodiment of the invention, the circuitry forcontrolling the timing of the switching of the current charging thecapacitor above may additionally include circuitry for comparing asignal indicative of the supply voltage to the electric circuit above toa signal indicative of an allowed voltage range for the electriccircuit.

Additionally, the electric circuit for controlling the timing of theswitching of the current charging the capacitor above may also includecircuitry for pulse width modulation of the transistor circuit.

In another embodiment of the present invention, a circuit for monitoringthe current in the circuit above could be added in order prevent theoccurrence of overcurrent in the electric hot-swap circuit above and todeliver a signal indicative of the current level in the electrichot-swap circuit to the circuitry for controlling the timing of theswitching of the current in the electric hot-swap circuit.

In the event of overcurrent being detected by said circuit, the currentcould then be switched off temporarily by the electric hot-swap circuitreceiving a signal indicative of overcurrent.

There are various ways of measuring of the current level in the electriccircuit of which a shunt resistor circuit is one non-limiting example.

One way of delivering the monitored signal indicative of the currentlevel in the electric hot-swap circuit the signal is to first amplify itand then to compare it to a reference value indicative of a maximumallowable current level in the circuit in a comparator and thereaftersending it to one of the inputs on the microprocessor circuit. Thiscomparison may be performed in the analogue or in the digital domain.

Naturally, an external circuit for monitoring the current level in theelectric hot-swap circuit is not necessary, since the same monitoringand measuring function could be built-in into the microprocessor itself.This would have the additional advantage of saving space on the circuitboard for an external current monitoring circuit and thereby productioncosts.

In another aspect of the present invention, the object of the inventionis reached by a method for providing hot-swapping of electronic boardsin an electric circuit, where the method comprises the steps ofproviding a current charging a capacitor on the electronic board,controlling an increase in the current charging the capacitor on theelectronic board, switching the current charging the capacitor andfinally controlling the timing of the switching for the current in theelectric circuit by means of a processing unit.

The switching of the current charging the capacitor on the electronicboard can include turning “on” and turning “off” a discrete transistor.

Optionally, the step of switching the current charging the capacitor onthe electronic board may include the monitoring the increase in currentin the above mentioned electric circuit.

The signal indicative of the current increase could be compared to acertain signal level indicative of the maximum current level in thecircuit, which would prevent overcurrent going through the circuitduring the insertion procedure of an electronic board.

In yet another aspect of the present invention a computer programcontrols a method for providing hot-swapping of electronic boardsaccording to the method described earlier, which comprises instructionsets for activating a supply voltage monitoring function in themicrocontroller, receiving a signal indicative of the supply voltage inthe electric circuit and switching a transistor in an electric circuitin response to the voltage signal.

Optionally, the computer program may include instructions for choosingbetween two modes for switching of the discrete transistor above:continuous switching, such as pulse-width modulated switching or justone switching cycle for the discrete transistor.

In a further step, an instruction set could be used to perform thecomparison between the measured signal indicative of the supply voltagelevel in the electric circuit on the electronic board and a signalindicative of the allowable supply voltage range in the electriccircuit.

In the event of said measured signal falling outside of the allowablevoltage range another instruction set may perform a watch dog resetoperation to restart the electric circuitry on the electronic board.

As an option, the computer program may include an instruction set formeasuring a signal indicative of the current level in the electriccircuit and comparing it to a signal indicative of a maximum allowablecurrent level in the circuit. In the event that the measured currentlevel exceeds the maximum current level in the electric circuit anotherinstruction set may perform a shutdown of the whole electronic boardmeaning cutting the electric circuit off from the power supply.

Alternatively, the instruction set may perform only a temporary shutdownof the electric circuit when a measured signal indicative of the currentlevel has reached a predefined maximum value. After a certain period oftime, the instruction set may restart the electric circuit again andwait until the next measured signal indicative of the current levelreaches the predefined maximum value and the same sequence would berepeated again.

The different aspects of the invention described above are describedwhen the electric hot-swap circuit is included onto an electronic board.However, the same electric hot-swap circuit could equally be installedinto a rack used in a telecommunication system where electronic boardsare inserted and removed from the rack.

However, the present electric hot-swap circuit is not only limited tothe use in telecommunication systems, but also in other technical areas,where a flexible, cost-effective and easily configurable hot-swapcontroller solution as provided by the present invention is applicable.

BRIEF DESCRIPTION OF THE DRAWINGS (OPTIONAL)

The invention will now be explained in more detail by a describingcertain embodiment thereof and by referring to the accompanyingdrawings.

FIG. 1 illustrates one example embodiment of the hot-swap controlleraccording to the present invention.

FIG. 2 illustrates the function of the microcontroller in the hot-swapcircuit in flowchart form.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows an example embodiment of a hot-swap circuit according tothe present invention. The hot-swap circuit in this example is locatedin a base station belonging to a mobile communication network. Clearly,the circuit could be located in any other location in a mobile or a datacommunication network or a server where the need for a hot-swapcontroller exists.

The example hot-swap circuit 100 in figure is connected to a negative48V supply 110 and comprises an auxiliary power supply 120, amicrocontroller 130, a current sense circuit 140, a switching unit 150and an LC-filter unit 150.

The reason for a negative voltage supply in telecommunication backplanesystems is to minimize the occurrence of corrosion in the presence ofcurrent leakage paths, because negative ions tend to corrode metals andthe negative ions are repelled by the negative supply voltage.

The hot-swap controller 100 comprises of an auxiliary power supply 120that converts the incoming −48V source voltage into more appropriatevoltages for electronic circuits, e.g. +10.5 V and +5V. The +10.5 Vsupplies a switching circuit 150, while the +5V supplies amicrocontroller 130 supervising the current switching process and acurrent sense unit 140 measuring the current in the hot-swap circuit100.

The voltage converter converting the incoming −48 V to +10.5V in theauxiliary power supply 120 can be for example of the type step downconverter.

On the other hand, the converter for the +5V uses the power from the+10.5 converter to power the microcontroller 130 and the current sensecircuit 140.

In this example, the +5 V, the converter is of the type LDO(low-dropout).

The voltage values in the example embodiment in FIG. 1 are mentioned forpractical purposes only and should not be interpreted as limitations.The power supply 120 might in other possible embodiments of the presentinvention equally convert other source voltages to other auxiliarysupply voltages according to need.

The microcontroller 130 in the hot-swap circuit 100 has the function ofsupervising the switching of the current in the circuit. It reacts uponthe signal from the current sense circuit 140 and switches the inputpower to the switching unit 150 on or off in order to limit the currentdue to the insertion of a large capacitive load into the circuit.

Also, the microcontroller 130 includes circuitry for measuring theactual level of the supply voltage 110 in the hot-swap circuit 100. Thereason for this is that when inserting an electronic board containingthe hot-swap circuit 100 into a rack, for example, the supply voltagemay at first deviate up to +/−15 V from the −48V supply voltage until ithas settled near the nominal −48V after a period of time.

The microcontroller 130 may be for example a microprocessor of the RISCtype or any other type of programmable processor. Also themicrocontroller 130 could be an ASIC, an FPGA or have some otherarchitecture suitable for the hot-swap application. However, theprogrammability of the microprocessor ensures the flexibility inchoosing the appropriate switching time for the switching on andswitching off the current in the circuit by a simple rewriting andreloading operation of the program code stored in the memory of themicroprocessor. One should mention however, that the programmability ofthe microprocessor is not required, since one could equally use amicroprocessor with a pre-programmed microcode.

The switching unit 150 according to this example embodiment of thepresent invention consists of a driver stage, a transistor stage and arectifier stage. The driver stage is a low-ohmic source for the gatecharge of the transistor in the transistor stage. The transistor in thetransistor stage is used to switch the input voltage to the LC-filtercircuit 160 depending on the signal from the microcontroller 130. In theexample embodiment in FIG. 1, the transistor used for switching theinput voltage to the LC-filter is a discrete to power transistor. Inother applications, where maximum currents in the hot-swap circuit aresubstantially lower than in telecommunication systems, integratedtransistor solutions could be used.

The rectifier stage in the switching unit is used as a discharge pathfor the inductor in the LC filter circuit 160.

Finally, the LC-filter circuit comprises a capacitor and an inductorcircuit. The capacitor is the energy storage to be filled during theswitching on phase of the power transistor in the switching unit 150. Itbasically represents the equivalent capacitance of the electronic board.The function of the inductor circuit is the free-wheeling of the currentcharging the capacitor.

The circuit in FIG. 1 may be located on an electronic board to beinserted in a rack situated in a telecommunication system, but couldequally be placed on the rack receiving electronic boards.

Also, it should be noted that the hot-swap circuit above is not onlylimited to the use in telecommunication applications, but in any othersystem where a hot-swap controller is needed or desirable. Thus theinventive hot-swap circuit could be used in those fields where insertionand removal of modules or electronic boards onto a common backplane isfrequent and where the switching off of the power feeding the backplanein order to provide for insertion and removal of electronic boards ingeneral is not desirable.

Now, an example embodiment of the method for providing hot-swapping ofelectronic boards in an electric circuit will be explained withreference to FIG. 1.

When an electronic board comprising the hot-swap circuit 100 from FIG. 1is inserted into a rack the electronic board is supplied by a currentcharging the capacitance of the electronic board (the equivalentcapacitance in the LC-filter circuit 160).

Since this charging current normally is a rapidly increasing currentreaching values in the tens of amperes range in a telecommunicationenvironment, its rapid increase is controlled by the power transistor inthe switching circuit 150, where the power transistor dissipates powerin the form of heat resulting from the current passing through it.However, in order to limit the amount of heat dissipated by the powertransistor of the switching circuit the increase in the current chargingthe capacitor on the electronic board is controlled by a microcontroller130 which switches the power transistor according to a methodimplemented for example as a software program operating inside themicrocontroller 130. By using, for example, pulse width modulation forswitching the power transistor in the switching circuit 150, the currentcharging the capacitor on the electronic board is stepped up slowly thuspreventing the building up of an inrush current which could potentiallyeither damage the electronic board or lead to malfunction of otherelectronic boards mounted in a rack in telecommunication systems due tothe initial high current drain following the insertion of the electronicboard.

It should be mentioned that the current level in the circuit could becontinuously monitored via measurement of a signal, e.g. a voltage,indicative of the current level in the hot-swap circuit 100 and comparedto a signal indicative of the maximum allowable current level. Thisfunction is performed by either an external current sense circuit 140 orin the microcontroller 130 itself. Also, the comparison between themeasured signal and the signal indicative of the maximum allowablecurrent in the circuit could be performed in the analog domain or in thedigital domain. In the analogue domain, the comparison may be performedby means of, for example a comparator, where the comparator produces ananalog output signal indicative of the difference between the twosignals on each of its inputs. This difference signal may then beconverted into a digital signal either at the output of the comparatoror at the input of the microcontroller 130. In the digital domain thecomparison may be performed as a logical comparison operation where themeasured signal is first converted into its digital representation andthen logically compared to another digital signal indicative of themaximum allowable current in the electric hot-swap circuit. This couldalso be performed either by an external circuit or inside themicrocontroller 130.

Also, the rising current resulting from when the electronic circuitboard is inserted into the rack of the telecommunication system iscontrolled by an inductor included in the LC.-filter circuit 160 duringthe time the power transistor in the switching circuit 150 is in the“ON” state.

When the power transistor is then set into the “OFF” state by themicrocontroller 130 due to a high current level in the hot-swap circuitbeing detected, the inductor “discharges” the stored energy via a slowlyrising current and charges the capacitor on the electronic board.

Next, method steps implemented in the microcontroller 130 in FIG. 1 willbe described with reference to FIG. 2.

At step 200 the microcontroller is powered on by the action of forexample inserting an electronic board into a rack in atelecommunications system (not shown). At the same the microcontrolleris reset.

At 210 the initialization of the microcontroller is performed so thatthe microcontroller starts its operation with correct initial values onits input.

Steps 200 and 210 are basically standard in any power on andinitialization procedure for a microcontroller.

Next, at step 220, the setting of the current sense function in themicrocontroller to on is performed which amounts to being prepared toreceive a signal on one of the terminals of the microcontroller wherethe signal is indicative of the current level in the hot-swap circuit.The signal indicative of the current level may be a voltage signaldelivered by the current sense circuitry 140 in FIG. 1.

Optionally at step 221 a check of the current level in the hot-swapcircuit may be performed via a measured signal indicative of the currentlevel where after it is compared to another signal indicative of themaximum allowable current level in the hot-swap circuit.

If this is the case, a shut down the power supply to the electronicboard at 222 is performed. In this case, the electronic board has to bere-inserted again to restart the hot-swap process and to supply it withthe voltage needed.

Furthermore, at step 230, a signal indicative of the supply voltagesignal 115 is compared to a signal indicative of the allowed supplyvoltage signal range in the hot-swap circuit.

The reason for this comparison is that the supply voltage to theelectronic board immediately after the insertion of the board into therack may vary for a time period around the nominal −48V supplied by therack. Thus, the instruction set at step 230 should allow for a slightvariation of the supply voltage around the nominal value for example by+/−15V.

The comparison operation above may be performed as a comparison in theanalog- or the digital domain and may be performed by an externalcircuit or inside the microcontroller 130.

If the signal indicative of the supply voltage signal 115 is outside theallowed voltage range, a watch dog/reset function is performed at step240, for temporarily switching off the power transistor in the switchingunit 150. The initialization step at 220 and the setting of the currentsense function at 230 then are performed again.

In the case that the voltage level of the measured signal indicates alevel within the allowed voltage range, a selection is made at 250 toeither switch the power transistor in the switching circuit 150 in FIG.1 in a continuous mode or perform a one time switching on function onlyfor the power transistor in the switching circuit 150.

In the example in FIG. 2, the continuous switching function is a pulsewidth switching function, but could of course be any other functiondepending on the application, the maximum current levels in the hot-swapcircuit to be expected and other factors.

Finally, the microcontroller performs switching according to a pulsewidth modulation scheme at step 260 controlling the power transistor inthe switching circuit 150 in FIG. 1. The pulse width D and the period ofa pulse T may thus be easily adapted. This is a significant advantageover existing solutions where one part of the hot-swap circuitry has tobe replaced.

Alternatively, at step 260, a one time switching operation of the powertransistor is provided.

A third variant (not shown) at step 250 would be the selection of a modefor continuous switching of the power transistor, where the currentlevel in the hot-swap circuit is measured for the duration of eachpulse. This would in practice entail turning the power transistor “on”by a voltage step, choosing to let the current in the hot-swap circuitreach a maximum allowed level as defined above and at the moment whenthe maximum value is reached, switch the power transistor “OFF” for atime period and switch it “ON” again, wait until the current rises to amaximum allowed level again, again switch the power transistor “OFF” fora time period and switch it back “ON” and so forth.

1. An electric circuit for minimizing in-rush current when hot-swappingan electronic board, wherein said electronic board comprises a circuitcharacterized by a series inductance and parallel capacitance, saidseries inductance limiting the rate of increase in input current to saidcircuit, said electric circuit for minimizing in-rush currentcomprising: a switch intermediate to an input voltage terminal and saidcircuit, said switch having a control terminal for selectivelycontrolling said switch between a conducting state and a non-conductingstate; a current sense circuit intermediate to said input voltageterminal and said switch; and, a processing unit coupled to said currentsense circuit and said control terminal of said switch, said processingunit selectively controlling said switch between said conducting stateand said non-conducting state in response to current flowing throughsaid current sense circuit, wherein a duration of said conducting stateis gradually increased as current charges said parallel capacitance ofsaid circuit.